This invention relates to the use of near infrared (NIR) reflectance spectroscopy for the monitoring of oxidation or weathering (degradation) of oil sand ores.
It is a common practice to commercially extract bitumen from oil sands using the hot water process. In the first step, called conditioning, the oil sand is mixed with water and heated with open steam to form a pulp. Sodium hydroxide or other reagents are added as required to maintain a pH in the range of about 8.0-8.5. After conditioning, the pulp is further diluted so that settling can take place. The bulk of the sand-size particles rapidly settles and is withdrawn as sand tailings. Most of the bitumen floats to the top to form a coherent mass known as bitumen froth which is recovered by skimming.
The oil sand is a quite complex material and includes sand grains, water, clay and bitumen filling the interstices between the sand grains. The concentrations of the components of the oil sands can vary quite widely throughout a deposit and, for instance, Thompson, U.S. Pat. No. 4,433,239 describes the use of near infrared for on-line monitoring of bitumen content in tar sands. For this purpose an infrared reflectance monitor was used with a first filter adapted to pass only wavelengths of about 2180 to about 2260 nm, absorbed by bitumen alone, and a second filter adapted to pass only wavelengths of about 2270 to about 2350 nm, not absorbed by any tar sand components. From a ratio of signals obtained an output is provided which is indicative of the bitumen content. The bitumen readings that were obtained were found to be essentially the same whether or not the tar sand had become dry and oxidized.
It is also known to use near infrared absorbance for measuring the octane of gasoline. This is described in Maggard, U.S. Pat. No. 4,963,745 where the octane number was determined by measuring absorbance in the t-butylmethyene band (1200 to 1236 nm).
It has been found that when oil sand ores become substantially oxidized or degraded, extraction of the bitumen is difficult. For example, a froth may be formed with an elevated mineral to bitumen ratio or there may be a reduced recovery of bitumen. This problem with the processing of oxidized or weathered oil sands is becoming a matter of serious concern as new mines are opened. Many deposits have only a very thin overburden and this results in the upper portion of the oil sand deposit being heavily oxidized, e.g. to a depth of as much as 12 meters. Also because of the activity of underground water, very deep portions of a deposit may also be oxidized. Intermediate portions of the deposits may have little or no oxidation. As such a deposit of oil sand is excavated and fed to a processing plant, e.g. on a conveyer belt, the degree of oxidation of the oil sands may frequently change. These changes have a serious affect on the processing if the processing conditions, amounts of reagents, etc. are not adjusted to compensate for the variations in the degree of degradation or oxidation.
A technique has been developed to quantify the degree of oxidation using microscopic examination of the froth produced. This technique involves creation of a froth sample and characterization of microscopic morphology of the bitumen. Oxidized ore produces a froth with a recognizable bitumen structure different from the unoxidized ore. Quantification of the degree of oxidation is then dependent upon examination of many froth samples and many fields of view to determine the relative amount of oxidation in the original ore. The correlation between the microscopic evaluation of oil sand ore oxidation and processing behaviour has been verified on a batch extraction scale, on a 4 tonne/hour pilot scale and with commercial scale extraction samples.
The above procedure is a complicated way of determining the degree of oxidation and it is an object of the present invention to find a way of on-line monitoring for the degree of oxidation of an oil sand ore and be able to use this information to automatically adjust the processing conditions.
The present invention in its broadest aspect relates to a unique method of using near infrared (NIR) reflectance for determining the degree of oxidation or degradation of oil sand ores. It has been found according to this invention that the degree of oxidation of an oil sand is not necessarily related to a particular NIR wavelength but can be related to certain aspects of NIR. Thus, it has been found that there are certain NIR wavelengths at which peaks of increasing or decreasing intensity correlate to increasing degrees of degradation or oxidation and that a general downward shift of the spectra baseline may also be correlated to increasing degrees of degradation or oxidation.
By testing a large number of samples using the above microscopic method and obtaining NIR spectra on the same samples, baseline shifts can be obtained indicative of the degree of oxidation of the ore. The degree of degradation or oxidation observed using the above microscopic method strongly correlates with processability of oil sand ores and patterns in the NIR spectra have been found which vary in proportion to the degree of oxidation observed microscopically. For instance, using the spectra baseline shift as the indicator, an ore with no oxidation will provide the highest baseline and an ore that is 100% oxidized will provide the lowest baseline. Using an NIR spectrometer, baseline shifts in NIR spectra can be correlated to the degree of bitumen oxidation or degradation in a wide variety of oil sands.
The baseline shifts can be used in accordance with this invention over a wide range of NIR wavelengths and, for instance, the spectral wavelengths available in commercially available on-line NIR oil sands bitumen measurement devices may be used. These may have spectral values such as 2120 nm, 1936 nm, 1836 nm, 2310 nm, 1723 nm, 2208 nm, etc.
The first region of the NIR spectrum showing a peak intensity relationship to degree of degradation or oxidation is at a wavelength of about 1150 to 1250 nm. In this region, there is a very significant increase in peak intensity with increasing degrees of degradation or oxidation of the oil sand ore.
The second region of the NIR spectrum that can be utilized for measuring the degree of degradation or oxidation is at a wavelength of about 1700 to 1800 nm. Here it has been found that the peak decreases with increasing degrees of degradation or oxidation. This decrease represents a loss of CH2 peak intensity.
The third region of the NIR spectrum that can be related to the degree of degradation or oxidation is found in the region of about 1900 to 2000 nm. Here the peak increases in intensity with increasing degrees of degradation or oxidation and this can be related to an increase in OH intensity.
The spectrometer is typically placed above oil sand to be monitored, e.g. above oil sand moving on a conveyor belt. The instrument continuously produces a measurement which is indicative of the degree of oxidation or degradation of the oil sand ore moving along the conveyor belt into an oil sand processing plant.
Costly chemical additions and/or processor changes, such as chemical additions and times for conditioning, feed rate or water to ore ratio, are necessary to control the processability of oxidized oil sand ores. By continuously monitoring the degree of oxidation and providing a signal indicative the degree of oxidation in accordance with this invention, it is now possible to continuously adjust the processing conditions to an optimum level thereby minimizing production costs.